1. Field of the Invention
The present invention relates to the treatment of materials in a fluidized bed reactor and particularly to the treatment of various sludges, spent liquors from pulping processes and other combustible materials.
2. Description of the Prior Art
It is well known that a fluidized bed reactor can be used for the incineration of various materials. The material is introduced into the bed of the reactor where it comes into contact with the hot fluidized particulate bed material. As an example of such an application the U.S. Pat. No. 3,319,586 can be cited, which describes incineration of waste sludges containing organic matters. Because of the high water content of these sludges after the mechanical dewatering process, the incineration of the sludges in a fluidized bed reactor involves problems.
By mechanical treatment it is, for instance, possible to remove so much water from sewage sludge that its solids content is about 20%. If appropriate chemicals are added to the sludge such as e.g. lime and ferricchloride a solids content of about 45% can be achieved, whereby the combustion in a fluidized bed reactor will be autogenous. If the solids content is lower, it is necessary to supply the reactor with auxiliary fuel in order to maintain combustion. From most industrial waste sludges it is possible to remove only so much water by mechanical means that the solids concentration will be lower than 20%. Incineration of such sludges causes high operating costs, which increase with the moisture content.
In order to improve the thermal economy of the incineration process, the heat of the combustion gases can be utilized for predrying the sludge either by direct or indirect heat transfer. In the direct method the combustion gases are brough into direct contact with the sludge and the exhaust gases will therefore contain malodorous gases. As the volume of the gas is large, the burning of the smelling components will cause considerable expenses. In the indirect method large heat transfer surfaces are needed, which causes high construction costs.
Canadian Pat. No. 524,796 shows an example of a method where the sludge is predried by contacting it with the combustion gases.
Earlier, the chemicals in spent liquors from the sulphate pulping processes have usually been recovered by burning the spent liquors in a recovery boiler, whereby sodium and sulphur contained in the digestion chemicals are recovered from the smelt in the form of sodium carbonate and sodium sulphide. The sodium carbonate is then converted into sodium hydroxide by causticizing the dissolved smelt. The combustion of the spent liquor takes place in three stages: drying, reducing combustion and oxidizing combustion. Because of this it is difficult to control the different stages carried out in the same space, in such a a way that the desired result will be achieved. This arrangement is too expensive for small pulp mills. A steam boiler is moreover not fully realiable in operation because it is subject to many disastrous boiler explostions.
The chemicals in the spent liquors coming from a sulphite pulping process can be recovered for instance as described in the Finnish Pat. No. 45.880 where the carbonate and the sulphide sulphur in the green liquor obtained from the smelt coming from the recovery furnace are separated, and the sulphide-containing solution is reacted with a bisulphite solution whereby the released hydrogen sulphide is converted by combustion into dioxide. The sulphur dioxide from the combustion process is reacted with a carbonate water solution in an absorption tower whereby bisulphite needed for the pulping liquor is formed.
Pyrolysis has been used for recovery of digestion chemicals from spent liquors, for instance in the SCA-Billerud process, where the sodium salts in sodium sulphite spent liquors are converted into sodium carbonate and the sulphur components into hydrogen sulphide. The hydrogen sulphide is converted by combustion into sulphur dioxide and absorbed into a sodium carbonate solution. This method is described in the Finnish Pat. No. 45.518.
It has also been proposed to treat the spent liquor in a rotating furnace. U.S. Pat. No. 3,787,283 describes a method for recovering chemicals, where a concentrated spent liquor from a sodiumbased pulping process is mixed with reactive alumina hydrate and formed into solid pellets by adding sodium aluminate. The pellets are fed into a rotating furnace where a temperature below the fusion temperature of sodium aluminate is maintained. A portion of the resulting sodium aluminate ash is dissolved in water and the solution is reacted with the sulphur dioxide-containing flue gases to form a slurry containing sodium sulphite, from which aluminum hydrate is separated. The remaining portion of the sodium aluminate ash is recycled and mixed into the spent liquor.
The operation of a rotating furnace involves several disadvantages. It is expensive and requires a great deal of maintenance. Its thermal economy is unsatisfactory because it is necessary to supply auxiliary fuel for maintaining the temperature required for the reaction. The heat transfer from the gas in the furnace to the treated material is, as is well known, poor. The furnace must therefore be large-sized. In order to reduce the dust losses the material has to be formed into pellets, for which reason auxiliary apparatuses are required before the furnace as well as after it. As a consequence of the pelletizing the solids content of the material supplied to the furnace must be high, which increases the operation costs.
It has also been proposed to incinerate spent liquors in fluidized bed reactors. As an example of an application of this kind, U.S. Pat. No. 3,635,790 can be cited. Because the combustion temperature has to be lower than the fusing temperature of the chemicals, for instance maximally 750.degree. C., when incinerating spent liquors containing sodium the combustion has to be performed within a temperature range where it is difficult to maintain stable combustion. The temperature can be reduced by feeding to the furnace a liquor which has a low solids content or by cooling the process with a great amount of excess air. In both cases the furnace has to be large-sized and is difficult to control.
Gasification of solid organic material has earlier been performed, for instance, by a method disclosed in U.S. Pat. No. 3,840,353, wherein a granulated carbon-containing fuel is introduced into a fluidized bed reactor and solid particles removed from the flue gases are returned to it. The combustion and the gasification of the carbon-containing material is brought about in the same reactor, for which reason it is difficult to control the reactions in a desired manner.
In order to avoid the aforesaid disadvantages, it has been proposed to subject the material to be treated to contact with hot material taken from the bed of the reactor before it is introduced into the fluidized bed reactor. A method of this kind is described in the German patent application No. 25 32 994. One of the drawbacks of the arrangement is that the hot bed material has to be transferred from the reactor and back to it and another drawback are the control problems connected with it when the quantity or the moisture content of the material to be treated changes. The wear of bed material and equipment also causes problems. Because thermal energy for evaporation of moisture in the material is taken from the bed, a corresponding amount of heat has to be transferred to it, i.e. the combustion must take place in the bed which therefore has to be of large volume. The supply to the bed must furthermore be distributed among several ducts, which will increase the construction costs.
The exhaust gases from the combustion chamber of a fluidized bed reactor contain fine material which can be separated, for instance in a cyclone separator. The fine material contains ash, fine particles from the bed material and usually also usuable chemicals. Its heat content is considerable. Depending on the velocity of the air flowing through the fluidized bed reactor, it will function differently. The higher the velocity is, the greater is the fluidized bed material entrained in the uprising fluidizing air and the more fine solids will be exhausted with the flue gases.